1. Field of the Invention
This invention relates to a method and apparatus for the direct, non-contact, sampling and detection of minute quantities of materials on surfaces.
More particularly, this invention is directed to a method and apparatus for impinging a plasma upon a surface being explored to create ions from materials on that surface, collecting the produced ions, and thereafter analyzing the ions to identify the material.
2. Description of Related Art
Military, security, and law enforcement concerns, as well as environmental monitoring and similar needs, all require a capability to sample and detect minute quantities of explosives, drugs, chemical and biological agents, toxic industrial chemicals and other compounds of interest on or in a variety of materials and surfaces. For most of those applications, it is extremely desirable that the analysis be performed with speed, accuracy, and on site.
Many of the chemical detection techniques and instruments in use for such purposes at this time rely upon the production and subsequent separation and identification of ions derived from targeted analyte chemicals. For example, among others, mass spectrometry, which utilizes ions to unambiguously identify analyte chemicals, and ion mobility spectrometry and differential mobility spectrometry, which compare the behavior of ions derived from the sampled chemical with libraries of characterized ions having known behavior. Such techniques are often preceded by sample treatment which can, for example, consist of the separation of chemicals in a complex mixture by chromatography or other techniques. The chemicals of interest must be ionized either before, during, or after such sample treatment and prior to detection and identification in a sensor having an output that depends upon some property of ions.
The ionization of chemicals can be accomplished by altering the molecular or electronic composition of the chemical through exposure to certain reagents, radioactivity, and/or heat. For example, many detectors use 63Ni to produce ions from chemicals in air. These ions are then directed to a sensor capable of detecting and identifying ions of interest and thereby providing information regarding the presence or absence of targeted chemicals. Other ways to produce ions include chemical reactions, ultraviolet energy, and thermal energy.
One limitation of such techniques has been the vapor pressure of the targeted chemical. For sensor technologies that are dependent on detecting ions in an air or gas stream, there must be a sufficient supply of targeted chemical molecules in air to produce enough ions to meet the threshold detection limits of such sensors. The detection of explosives is a case in point. The saturated (air) vapor pressures of explosives range over at least seven orders of magnitude. This means that air around different explosives contains some, little or virtually no molecules of these different explosives. The consequences of such dependences of a detection technology on vapor pressure are that some explosives are detected, others detected poorly, and some not detected at all. Various techniques have evolved over time to deal with this deficiency. For example, chemicals can be concentrated from air using polymers or filters, or solid particles can be gathered on filters by vacuum methods. Subsequent heating of such filters or polymers to vaporize the entrained chemicals can result in sufficient chemical in vapor form for ionization and subsequent detection. However, these techniques require additional equipment and consumables (preconcentrators, filters, wipes, heaters), time, and operator training. These factors increase the cost of detection and reduce the number of detections that can be accomplished per unit time. They also introduce a variable into the results related to the adequacy of training and attention to protocol of the individual performing the procedures.
A means to directly ionize chemicals on surfaces, as well as in air, would eliminate the need for time-consuming and expensive multiple step sample collection and ionization procedures. Such a means has been described in commonly assigned patent application Ser. No. 11/122,459. In that application means were described whereby ions and energetic species produced in a gas discharge were then carried in a gas stream that was directed upon a target surface to subsequently ionize chemicals on that surface or in air in proximity to the surface. This technique was found to greatly reduce the dependence of detection on target chemical vapor pressure. For example, explosives having saturated air vapor pressures ranging over seven orders of magnitudes were detected approximately equally well, and in less than four seconds, using this technique.
The invention described in this application provides a new and different approach to ionizing target chemicals on a surface through use of a low to moderate temperature, atmospheric, or near atmospheric, pressure plasma plume that is projected directly upon the surface to create ions which are then collected and identified.